Sound ranging system



g 1932- H. w. LAMSON 1,874,196

scum) ameme SYSTEM Filed July 16. 1928 3 Sheets-Sheet 1 Aug. 30, 1932. H. w. LAMSON 1,874,196

SOUND HANGING SYSTEM I Filed July 16, 1928 5 Sheets-Sheet 2 Aug. 30, 1932. 'H. w. LAMSON 1,874,196

scum) RANGING sYsTsu Filed July 16. 1928 3 Sheets-Sheet 3 I TTOQA/E) Patented Aug. 30, 1932 UNITED STATES PATENT OFFICE HORATIO W. LAMSON, OF ARLINGTON, MASSACHUSETTS, ASSIGNOR TO GENERAL RADIO COMPANY, OF CAMBRIDGE, MASSACHUSETTS, A CORPORATION OF MASSACHUSETTS SOUND HANGING SYSTEM Application file'd 1| uly 16,

My invention relates to a method for and a system in which a variable retardation of 'be binaurally off-center.

electric currents is effected and has particular reference to the application of the above method and system in sound ranging systems. Sound ranging is the determination of the direction from which a sound is coming in whatever medium it may happen to be traveling. Such systems, as a rule, operate on the principle of binaural compensation.

To locate the direction or range of a sound,

living creatures includin human beings, as-

a rule, turn their heads in such away that their ears are substantially equi-distant from the source of sound. If one ear is farther from the source ofsound than the other, the impulses in the farther ear will be translated into conscious impulses later than those from the other ear and thus the sound images will Unconciously the creature or person turns his head so that the difference in time is decreased to zero, making the line joining the two ears perpendicular to the direction of the oncoming sound.

Systems have been arr'angedworking on substantially the same principle wherein a pair of sound receiving instruments, either mechanical or electrical, have been disposed in a medium, commonly air or water, for the ranging of sound. By suitably connecting the sound receiving instruments to the ears of a person so that oncoming impulses from each of the receiving instruments will be con+ ducted to the ears of a person, it is possible to range a sound by turning the two receiving instruments together and thus centering the 1928. Serial No. 293,228.

images. Although this binaural centering of two sound images from a common sound source involves synchronization, it is distinctly different from ordinary synchronization. Thus for example the ordinary erson can not resolve two sounds that reach hlS cars at a smaller time interval than one onehundredth of a second. ,Such closely timed sounds willappear as continuous to the person. However, when ranging sound, the ordinary person can determine the direction of sound travel to a degree correspondin to one one-hundred thousandth of a second ifference in time of arrival between theimpulses at the two ears. Hence it is obvious that the binaural centering of two sound images from one sound source, really a synchronization of two images from a common source, is fundamentally different from ordinary synchronization of separate sounds. In sound ranging systems, it is the binaural centering faculty which is made use of. The variation in the length of acoustic paths from the sound collectors or pickups to the ears of the operator merely cause a virtual binaural centering of the sound images. By equatin the virtual centering action with the actua physical accentricity of the sound source to the sound collectors, it is possible to directl determine the true direction of sound trave Sound ranging systems of the electrical variety and employing the principle of an artificial lag or binaural compensation are substantially as follows. A pair of sound receiving instruments or pickups is disposed in the medium in which the sound is travcling at a predetermined distance apart from each other and preferably facing in the same direction. These pickups, although of various types, are usually microphones. Connecting the two microphones is an artificial retardation or lag line composed of a plurality of low pass filter sections. Each sec-' tion of such a line is usually composed of an inductance unit and a capacity unit, the inductance unit being adapted to be in series with the line, while the capacity unit is across the line. A complete lag line of this character usually comprises a plurality of inductance units connected in series and a plurality of capacity units connected at the junctions of the inductance units across the line. Such aline has the property of delaying an electric wave without substantially changing the phase relationship between the current and the voltage.

The value of the retardation of the entire line connecting the sound receiving instruments is usually equal to the time interval within which a sound impulse could travel the distance separating the sound receiving instruments in the medium considered. To use such a system, it is customary to sever the retardation line at any one of the inductance unit junctions. is thereupon placed across the line on one side of this break while another receiver is similarly connected on the other side of the break. The result is that each sound receiving instrument has a predetermined amount of lag line connected between it and its corresponding telephone receiver, the two portions of the lag line being complemental to each other. By severing the lag line at any one of the various junctions of the inductance units, it is possible to distribute variable portions to each part of the system. In this way, it is possible to artificially compensate in the electrical lag line for the unequal arrival of sound impulses at the two receiving instruments and thus obtain, by this compensation, binaurally centered .impulses at the receivers. By calibrating the point of severance of the lag line with respect to each end of the line, it is possible to directly read off in degrees the angle between the base line and the direction of travel of the sound waves. An operator listening at the two telephones can thus arrange the complemental portions of the lag line so that the impulses in the telephones will be synchronized, thus binaurally centering the sound images.

To sever the line at any one of a number of inductance unit junctions and connect a telephone receiver to the lag line on each side of the break, requires a very complicated system ofswitching, both with regard to the electrical and mechanical features. Although in order to simplify the switching problem, the lag line may be composed of a small number of sections, each section having a high unit value, certain undesirable fac tors enter in when this is done. If the sections are made large in value, the operation of the lag line is fundamentally affected in an undesirable manner. The cut-ofi' frequency of the lag line, which is at the same t1me a low pass. filter, becomes so low that the utility-of the system becomes restricted to signals in which extremely low frequencies are used. Furthermore, the only place at which the line may be severed is at the junction points of each section, that is, across each condenser. .It'is manifest that if large A telephone receiver sections are to be cut in and out as units, it will be impossible to obtain any more than a very coarse variation of length of lag line.

Since the variation in the length of lag line corresponds in a physical sense to a change in the direction of sound, it is equiv alent to dividing up the angular range in which the device will operate into an arbi trary number of possible directions with no intermediate values. Electrically, any reading, intermediate indications, is impossible since there is no correspondence between the position of the indicator and connections in the system.

In order to obtain a high degree of accuracy in sound ranging, it is customary to increase the number of sections composing the lag line, making eachsection of smaller unit value. The switching problem along with the complexity of wiring and number of junction points, becomes enormously increased, making an accurate system of this kind very complicated and expensive. Furthermore, even in such a system, however small each section of the lag line might be made, it is still necessary to make distinct jumps from one section to another. Although much finer readings are possible, interpolation between readings is just as impossible and just as meaningless as in a less complicated and cruder system.

An object of my invention is to devise a lag line, the effects of whichcan be varied to as fine a degree as may be desired. Other objects are to effect such variation in lag line in a smooth, gradual, and uninterrupted manner, and to obtain such variation without any accompanying complexity of switching or connections. Further objects are to incorporate such a la line in a system such as for ranglng soun to make such a system smoothly and uninterruptedly operable over its entire range; to allow of interpolation between any two dial indications; and to devise a sound ranging system of this character which will be simple, inexpensive, and easy to operate.

In general, my improved lag line is composed of a nominal number of ordinary filter sections of well known construction. These need not necessarily be of equal lag value. In order to uninterruptedly vary the effective lngth of the line, I obtain currents at potentials corresponding to the potential in any To obtain such potentials, I dispose a pickup coil in inductive relationship to any de-.

sired inductive units or portion thereof of the lag line. The electrical potentials induced in this pickup coil, by virtue of the several current nnpulses in the inductance units coupled thereto, have of course, definite lags with respect to the electrical impulses entering the initial input section of the line. The net or summation potential induced in the pickup coil will then have a definite lag with respect to the impulses entering the input of the line which lag may be considered as the vectorial summation of the several lags corresponding to the impulses traversing the individual inductively-coupled inductance units.

By arranging the inductance units and coil in such a manner that selective cooperation may be obtained between the coil and any portion of the inductance part of the lag line, it is possible to obtain any desired retardation within the range of the entire lag line. If now thepickup coil is so moved with reference to the lag line that it comes into closer inductive relationship with inductance units ahead in the direction of motion and at the same time reduces its inductive relationship with the inductance unitsin a direction opposite to its motion, then the net induced potential in the pickup coil willhave a time lag with respect to the input ofthe lag line, which lag will increase or decrease" asthe case may be, not by discrete discontinuous intervals but in a perfectly continuous, uniform and uninterrupted manner. The lag of this net induced potential will, of course, change by an amount equal to the lag of the closest associated section of the line when the pickup coilhas been displaced a distance corresponding to the physical dimensions-of said section. In this manner the lag of the net induced potential corresponding to any position of the pickup coil may be determined from said position of the coil and the constants of the associated lag line.

' Due to the interplay of the capacitance and inductance on electrical waves, the gradual variation in length along the inductive portion of the lag line: has an effect on the effective length of the capacitative portion of the lag line. The mere fact that a variation in effective length of less than the value of one inductive unit is made, does not indicate that the capacitative portion of the system remains unchanged. Each inductance unit may be considered as a'connecting link between adjacent capacitative units and change of the effective length of the inductance portion of the line from a point just in advance of the junction point where the capacitative unit is connected to a point just beyond this junction point, does not cause any sharp variation in the effective length of the line. Hence, while physically only the inductive portion of the lag line is varied, electrically, the capacitative portion .of a vacuum tube amplifier.

of the lag line is also varied in the same uninterrupted manner because of the interaction between the two portions of the lag .line.

In order to obtain real precision in the alteration of the effective length of the lag line, it is highly desirable-that no nodal points exist within the line. Such points are caused by reflected waves interfering with the oncoming waves and causing so called standing waves. In order to eliminate any such possibility, I preferably avoid having any sharp electrical discontinuity in the line. At the end ofthe lag line, I dispose a terminal characteristic impedance of any kind whatsoever. As is well known, such an impedance has a value equal to the value of the line facing it. Within this impedance all the electrical energy coming through the line is absorbed without reflection. If the coil which is in inductive relationship to the inductance units of the lag line, were allowed to have secondary currents therein induced by the currents in the inductance portion of the lag line with which it cooperates, it is obvious that there would be highly undesirabletransformer action. If this pickup coil were allowedto absorb energy from the lag line, it would constitute a marked departure at that portion of the line from the ordinary characteristics of the line and result in reflected waves or ripples being sent out to both "ends of the line. In .order to avoid such undesirable possibility, I preferably arrange so that the potentials induced in the coil set up potentials in another circuit in which currents may flow without reacting on the coil. To obtain this, I preferably couple the coil to the input side By properly biasing the grid of the amplifier, it is possible to obtain output amplified potentials and currents without having any appreciable current flowing in the pickup coil. In this .way, it will be seen that original impulses incoming into the lag line will be dissipated, but at any desired degree of retardation, may give rise 'to potentials which in turn give rise to currents.

In order to apply such a lag line to a sound 3 ranging system, I preferably dispose a complete lag line in operative relationship to each of the two sound collecting or receiving instruments commonly known as pickups. Preferably the lag lines are equal in length or retardation value, each one having a retardation value somewhat equal to the time which a sound impulse takes to travel bctween the two sound collecting instruments in the medium desired. Each of the lag lines has its pickup or secondary coil in variable relationship to the inductive portions of said lines. Each of said coils is connected to a suitably biased thermionic amplifier, the outputs of which go to indicating means such as telephones. The net time lag between the responses of the two indicating instruments will depend upon the time lag in the excitation of the sound receivinginstruments and the compensating lag in the effective portions of the two lines. It is evident that the difference in the value of the effective lengths of the lag lines will determine the compensation of the current impulses.

By suitable mechanical means, I arrange the two coils so that their positions are simultaneously and correspondingly changed, but in an opposite sense with respect to each of the lag lines. Thus as one coil is at the input end of the lag line, the other coil is at the terminal impedance end of its line. Then as the first coil moves away from the input end of the line and toward the impedance, the other coil moves away from the impedance of its line and toward the input end. At the center of the lines, the coils will be in the same position on the lag line and there will be no net compensatory lag introduced.

Such a position of the coils will correspond to a direction of sound perpendicular to the line joining the sound receiving instruments.

The mechanical means actuating both coils simultaneously may be suitably arranged to actuate any desired ind cating means. Such means may be calibrated in any terms desired and usually would be calibrated in terms of degrees giving the angle between the direction of sound travel and the base line at the sound receiving instruments. It is evident that the indications may be made as close or as far apart as may be found desirable since interpolations between indications. are possible. r 7

Although such a sound ran ging system maybe read to as fine a degree as may be found desirable, the simplicity of the system as well. as expense are practically independent of the vrccisi-on, except for ordinary factors like workmanshipand care in the.manufacture.

In another aspect of my invention, the variable line may be considered as a low pass -filter whose cut off slope may be gradually and smoothly varied from a gentle taper to a sharp fall. tions of like character are added. the cut-off of the filter tends to become much more sharp and critical by definite steps. For some purposes, it may be desirable to gradually vary the slope of the cut-offby varying the effectivelength of the filter. Of course. the original currents through the filter will be dissipated but currents having the desired properties will appear at the output of the'amplifier. The sections of the filter need not necessarily be of similar character and might beso designed as to actually vary the cut-off frequency. In such a case, it is possible to obtain a smooth and gradual variation of the cut-off frequency.

Although I have disclosed my improved In filters of this. kind, as sec lag line in conjunction with a sound ranging system, its field of utility'is in no way to be circumscribed thereby. Thus it is possible and may be found desirable in the recording and reproduction of sound to introduce echo effects and this may be accomplished by means of my improved lag line to any desired degree. Another field to which my invention may be applied is in radio direction finders or radio ranging systems. It is obviously possible to replace the spaced pickups by spaced radio receiving systems. Other applications of the lag line will occur to those skilled in the art, all of which are within the scope of my invention.

Referring to the drawings, Figure 1 is a diagrammatic showing of my invention as applied to a sound ranging system;

Figure 2 shows a mechanical embodiment of the diagrammatic showing of Figure 1 Figure 3 is a modification.

Referring to Figure 1, 1 and 2 are sound receivers of any type whatever, such as microphones, and adapted to translate sound impulses into electrical impulses. These receivers as used generally are so disposed that they can pick up sound impulses traveling through water or air, and are separated by a predetermined distance. Battery 3 energizes the microphones, the circuits of which are completed through the primaries of transformers 4 and 5. The use of transformers at these points is optional.

The secondaries of these transformers are connected to artificial lag lines 6 and 7 respectively. These lines are composed of inductance units 8 connected in series on one side of the line and so disposed in this instance that there is the proper amount of mutual inductance between adjacent units. Condensers 9 are connected from the junctions of adjacent coils across to the other side of the line, which may be composed of as many sections as may be found desirable. Across the further end of each line is connected a terminal characteristic impedance or network 10, the value of which is such that the impedance at the terminals of 10 looking either way, is the same. In this way, electrical impulses starting in transformers 4 and 5 go down the lines, being retarded therein and are eventually entirely absorbed in impedances 10. The absorption is such that no reflections occur. 1

In inductive relation to the inductance units 8 of artificial lines 6 and 7, are pickup coils 11 and 12. These coils are connected respectively to the grids of vacuum tubes 17 and 18. The other ends of the coils are for the two tubes and is connected to the plate of each of the tubes through'conduc- 23 and 24. If desired, individual plate batteries may be used for each tube. The secondaries of transformers 23 and 24 are coupled to an indicating means, here shown as telephones 75. and 76 by leads 77 and 78.

Across the secondaries of transformers 23 and 24 is conductor 90 with switch 79. J oining the point between telephones 75 and 76 and between the two secondaries of transformers 23 and 24 is conductor 91 with switch 80. The reasons for the presence of the switches and cross connections will appear later.

Coils 11 and 12 are gradually movable from one end of the line to the other. The arrangement is such that "as coil 11 moves toward impedance 10 of line 7, coil 12 moves away from impedance'10 of line 6. Artificial lines 6 and 7 are preferably of equal value. The time lag value of lines6 and 7 is such that the retardation suffered by electric currents through the entire line is somewhat greater than the time taken for sound waves to travel the distance between microphones 1 and 2 in the medium.

The operation of the system is as follows Assume a source of sound directly in front of transformers will start out down the artificial lines at the same instant. The impulses from transformer 4' will go down the entire length of line'6 and suffer the maximum time lag before coil 12 will be affected. The current impulses in the last sections of line 6 will.

induce an electro-motive force in coil 12. By biasing the grid of tube 18 with a battery 14 of suitable value, it is possible to prevent anycurrent from flowing in coil 12, thus elimin ating the absorption of any energy from the lag line. The potential impulses induced in 12 affect the grid of tube 18 and give rise to current impulses through primary of transformer 24. Induced impulses in't'he secondary go through lead 7 8. through telephone 76, switch 80, lead 91. 0 Thus the impulses in telephone 7 6'are delayedby nearlythe full lag of line 6 with reference to the microphone 2.

Impulses from transformer 5 starting -down line 7 immediately induce an electromotive force in coil 11. Although the impulses from transformer 5 travel down the result is that impulses due to coil 11 will an rive at telephone 75 sooner than impulses due to 12 will arrive at telephone 76. The difference between these artificial lags will be approximately equal to the time lag suffered by the sound waves traveling the distance between the pickups through the medium. The listener at telephones 75 and 76 will thus be audibly warned that coils 11 and 12 are not correctly positioned since the sounds in the telephones are not synchronous and the sound image will be binaurally ofi' center.. He will, therefore, in the case cited, operate coils 11 and 12 so that one or both will approach electrically identical points on each line, whereby the sounds will become synchronous, because the artificial lags will then be equal, that is, the compensating lag will be zero. By suitably calibrating the positions of coils 11 and 12 with respect to the end positions as shown, it is possible to directly read off the angle between the line joining microphones I and 2 and the direction of travel of sound in the medium.

To cause the sound impulses to arrive at telephones 75 and 76 at the same instant, that is, to obtain an exact binaural centering of the image of'the sound source, coils 11 and 12 may be moved as little asmay be found desirable. Since the pick-up coils 11 and 12 of each of the lines are so related to the lag lines that there is mutual induction between themand several inductive units of the respective line, it is impossible for coils 11 and 12 to undergo any sharp and discontinuous variation in the effective potentials induced therein. Thus the gradual change of position of the coils 11 or 12 with respect to the inductive portions of the lag line results in a smooth and uninterrupted change in the effective lag between the microphone 1 and telephone receiver 75 and between microphone 2 and telephone receiver 76.

Referring to Figure 2, the inductance units of the lag lines, coils 8, are disposed in insulating tubes 25. The coils are separated by spacing blocks 36 to give the desired amount of mutual induction. An axial channel allows connecting wires from the junctions of coils 8 to come out in a cable 26 for connection to a panel 27. A group of condensers 9 forming the capacity units of the line are properly connected to panel 27 so that the artificial line is finally connected as shown in Figure 1. Around the outside of tubes 25 are pickup coils 11 and 12 wound on suitable forms and adapted to slide the length of tubes 0 25. As shown, these pickup coils are longer than the length of any one inductanceunit and preferably are long enou h so that they include portions of each of the adjacent units. In this instance, coils 11 and 12 are long enough to cover three of coils 8.

Tubes 25 are suitably mounted in a frame 35. This frame supports two sprocket wheels 32 and 33 connected by an endless sprocket chain 31.. The chain is afiixed to movable coils 11 and 12 by suitable means i 30 so that as the chain moves, the coils 11 and 12 move with it. The coils are connected to proper points on the chain so that by turning sprockets 32 and 33,coil 11 will go one way and coil 12 will go the other way. By suitably connecting the shaft of sprocket 32 or 33 to a suitable indicating dial, the angle between the direction of sound travel and the microphone base line may readilybe read off. Coils 11 and 12 are connected by flexible leads 37 to batteries and tubes as shown in Figure 1.

Figure 3 shows a slightly modified form. As is obvious from Fi ure 1, it is possible to reverse one of the arti cial lines so that pickup coils 11 and 12 may travel in the same direction instead of in opposite directions. The

artificial lag lines are reversed so that coils 11 and 12 are connected by cross arm 40 and may slide together. This sliding may be suitably effected by sprocket chain 42 connecting cross arm 40 at 41 and suitably actuated by sprockets 43 and .44. The whole device is mounted in a frame50. Sprockets'43 or 44 may actuate an indicator or a dial so that angles may be directly read off.

Thus far the system has been assumed to be connected asshown in Figure 1. However, by suitably operating switches 79 and 80, the system may be made to operate so that instead of synchronizing impulses, or binaurally centering continuous impulses, the

\ setting of the pickup coils may be made with the object of obtaining a maximum or minimum amount of sound. If switch 79 is closed while switch 80 remains closed, conductors 77 and 78 will always be at the same potential. The net result of this will be that whateveriiisplacement of sounds there might normally be between telephones 7 5 and 76, will be eliminated and the same sounds will issue from both telephones simultaneously. The sound issuing from both telephones will now be a vectorial summation of'the energies in transformers 23 and 24 and will represent an averaging, rather than a separation, as before. It is evident that, if the net compensation in the lag lines is equal and opposite to the difference in arrival of the sound impulses at thepickups, the sound impulses issuing from telephones '7 5 and 76 will be exactly in phase and at a maximum. This is because the phase difference having been reduced to zero, the addition becomes merely arithmetvalue.

ical and results in a maximum absolute At'this point, the setting of the pickup coils will correspond to a binaural centering of the images of sound before and the direction of sound will be determined.

If switches 79 and 80 are both open, the outputs of transformers 23 and 24 will be in opposition through telephones am]. 76. If

the pickup coils are properly positioned with a given direction of sound travel, then telephones 7 5 and 76 will be silent. This is because the two outputs are in direct opposition and the summation reduces to mere arithmetical subtraction, resulting in a minimum absolute value. It is perfectly obvious that this same result may be obtained by having both switches 79 and 80 closed, as above, and reversing the polarity of one of the pickup coils.

Although I have shown the inductance units of the artificial lines as mounted in.a straight line, it is obvious that they may be bent into a circular shape, forming a toroidal structure. Coils 8 might also have their axes radial as the coils of a generator. The secondary coils could then be mounted similar to the rotor coils of a generator. My invention isin nowise limited in the disposition of the elements of the lag lines or to the mechanical means employed for moving the pickup coils or recording the motion thereof.

by any distortion in the inductance units of said line due to the presence of said inductively associated means is prevented.

2. A sound ranging system comprising a plurality of pickups adaptedv to translate sound impulses into electrical impulses, means for translating electrical impulses into sound impulses, an artificial line COID- posed of interconnected inductance and capacity units intermediate said pickups and translating means, means inductively associated with the inductance units of the artificial line, said inductively associated means being connected to the translating means, said connections including a vacuum tube so biased that no appreciable input current is required, for preventing any appreciable distortion in the inductance units of said line due to the presence of said inductively associated means and means for changing the inductance units of the artificial line with which said'inductively associated means cooperates.

a 3. A sound ranging system comprising a pair ofpickups adapted to translate sound impulses into electrical impulses, a pair of equal and separateartificial lines, means for emme generating impulses in said lines corresponding to outputs of said pickups, means for translating electrical impulses into sound impulses, electro-magnetic coupling means be-' tween said lines and said translatipg means and means for keeping the current inpaid coupling means at a negligible value 4. In a sound ranging system, an artificial line comprising a plurality of interconnected inductance and capacity units, said inductance units being so arranged that there is mutual inductance between adjacent units, a coil in inductive relation to the inductance units of said line, and means for keeping the current in the coil at a negligible value.

5. An artificial line comprising inductance and capacity units connected together, means inductively associated with predetermined portions of the inductive portion of the line for uninterruptedly varying the eifective lag value of the line, said means including means for keeping the current in said inductively associated means at a negligible value.

6. An artificial line comprising inductance and capacity units connected together and adapted to retard electric currents, means for obtaining potentials corresponding to currents within said line and having any desired degree of retardation within the range of said entire line while keeping the current in the said means at a negligible value, and means for obtaining currents corresponding to said potentials.

7. An artificial line comprising inductance and capacity units connected together and adapted to retard electric currents, a terminal characteristic impedance at one end of said line, means for obtaining potentials cor responding to currents'within said line and having any desired degree of retardation within the range of said entire line and while keeping the current in the said means at a negligible value, means for obtaining currents corresponding to said potentials.

8. A retardation system comprising a plurality of inductance and capacity units connected together to form an artificial line, a characteristic impedance at the end of said line, a coil associated with predetermined in ductance units, and a thermionic amplifier connected to said coil, said amplifier being so adjusted that it functions substantially as a potential amplifier only.

9. An electric system comprising a plurality of pickups adapted to translate sound impulses into electrical impulses, means for translating electrical impulses into sound impulses, an artificial line composed of inductance and capacity units interconnected intermediate said pickups and translating means, means inductively associated to coop' mate with more than one of said inductance units at one time, said inductively associated means being connected to the translating means, said connections including means. for

keeping the current in the inductively associated means at a. negligible value, and means for changing the inductance units of the artificial line with which said inductively associated means cooperates.

10. An electric system comprising a plurality of pickups adapted to translate sound impulses into electrical impulses, means for tranlsatingelectrical impulses into sound impulses, an artificial line associated with each of said pickups, said artificial line having a dead end composed of a terminal characteristic impedance, and means inductively associated with the inductance units of said line, said inductively associated means being connected to the translating means, and said connections including means for keeping the current in the inductively associated means at a negligible value.

11. An electric system having, in combination, two sound pickup devices, means for translating the sound impulses picked up by each pickup deviceinto electrical impulses, lag lines interconnected intermediate the pickup devices and the translating means, means inductively associated with each lag line for picking up voltage impulses from the corresponding lag line, means for uuinterruptedly varying the effective value of the corresponding lag line in order gradually and smoothly to retard the voltage impulses picked up from the corresponding lag line, means whereby the inductively associated means draws a negligible amount of energy from the lag lines,

- a circuit connected with each inductively associated means, and means for making the retarded impulses manifest in the two last named circuits.

12. An electric system having, in combina tion, two sound pickup devices, means for translating thesound impulses picked up by each pickup device into electrical impulses, lag lines interconnected intermediate the pickup devices and the translating means, each lag line having a plurality of inductor coils, a pickup coil for each lag line adapted to be moved relatively to the inductor coilsof the corresponding lag line, whereby ad acently disposed inductor coils of each lag line serve successively as primary coils and the corresponding pickup coil serves as the secondary coil of a transformer to cause each pickup coil to pick up voltage impulses from its corresponding lag line, means for continuously moving relatively the inductor CO1lS and the corresponding pickup coil of each line to vary uniuterruptedly the effective value of the corresponding lag line in order gradually and smoothly to retard the volt-* age impulses picked up from the corresponding lag line, each lag line having a terminal characteristic impedance for maintaining the,

impedance of the corresponding lag line constant in order to prevent reflections of current waves within the lag lines, two vacuum tubes each having an input circuit and an outtive input circuits, whereby the retarded impulses are made manifest in the output circuits.

i 13. An electric system having, in combination, two sound pickup devices, means for translating the sound impulses picked up by each pickup device into electrical impulses, lag lines interconnected intermediate the pickup devices and the translating means, means inductively, associated with each lag line for picking up voltage impulses from the corresponding lag line, means for uninterruptedly varying the efiective value of the corresponding lag line in order gradually and smoothly to retard the voltage impulses picked up from the corresponding lag line,

two vacuum tubes each having an input circuit and an output clrcuit, means for biasing the vacuum tubes to keep the current flow in the input circuits at a negligible value, and means for connecting the inductively associated means in the respective input circuits, whereby the retarded impulses are made manifest in the output circuits.

14. An electric system having, in combina .tion, two sound pickup devices, means for translating the sound impulses picked up by each pickup device into electrical impulses, lag lines interconnected intermediate the pickup devices and the translating means, each lag line having a plurality of inductor coils, a pickup coil for each lag line adapted to be moved relatively to the inductor coils of thevcorrespondin lag line, whereby adjacently disposed inductor coils of each lag line serve successively as primary coils and the corresponding pickup coil serves as the secondary coil of a transformer to cause each pickup coil to pick up voltage impulses from its corresponding lag line, means for continuously moving relatively'the inductor coils and the pickup coil of each lag line tU vary uninterruptedly the effective value of the current flow corresponding lag line in order gradually and smoothly to retard the voltage impulses picked up from the corresponding lag line, two vacuum tubes each having an input circuit and an output circuit, means for baising the vacuum tubes to keep the in the input circuits, at a negligible value, andmeans, for connecting p the pickup coils with the respective input circuits, whereby the retarded impulses are made manifest in the output circuits.

15. An electric system having, in combination, two lags lines, means for obtaining electrical currents in each lag line, means inductively associated with each lag line for picking up voltage impulses from thecorrespondlng lag line, means for uninterruptedly varyreversaing the efiective value of the corresponding lag line in order gradually and smoothly to V trical currents in each lag line, each lag line having a plurality of inductor coils, a pickup coil for each lag line adapted to be moved relatively to the inductor coils of the corresponding lag line, whereby adjafcently disposed inductor coils of each lag line serve successively as primary coils and the corre-- sponding pickup coil serves as the secondary coils of a transformer to cause each pickup coil to pick up voltage impulses from its 'corresponding lag line, means for continuously moving relatively the inductbr coils and' the corresponding pickup coil of each lag line to vary uninterruptedly the effective value of' the corresponding lag line in order gradually and smoothly to retard the voltage impulses picked up from the corresponding lag line, each lag line having a terminal characteristic impedance for maintaining the impedance of the corresponding lag line constant in or- 1 der to prevent reflections of current waves within the lag lines, two vacuum tubes each having an input circuit and an output-circuit, means for biasing the vacuum tubes to keep the current flow in the. input circuitsat a negligible value, and means .for connecting the pickup coils with the respective input circuits, whereby the retarded impulses are made manifest in the output circuits.

17. An electric system having, combination, two lag lines, means for obtaining electrical currents in each lag line, means 1nductively associated with each laghline for picka e corresponding up voltage impulses from 1: ing lag line, means for uninterruptedly varyingthe efiective value of the corresponding lag line in order gradually and smoothly to retard the voltage impulses picked up from the corresponding lag line,'two vacuum tubes In testimony whereof, I have signed my I name to this specification this 13th day of July, 1928. a

" HORATIO a. LAMSON.

CERTIFICATE or CORRECTION.

1 Patent No. 1,s74, 196. August 30, 1932.

HORATIO w. LAM'SON.

It is hereby certified that error appears in the printed specification of the above-numbered patent requiring correction as follows: Page 1, line 78, for "accentricity" read "eccentricity"; page 2, line 118, for "lngth" read "length"; page 5, line 64, for "7" read "17''; page 7, line 41, claim 7, strike out the word "and", and insert the same before ,"means" in line 43; page 8 line 86, claim 16, for "coils" read "coil"; and that the said' Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 29th day of November, A. D. 1932.

M. J. Moore, (Seal) Acting Commissioner of Patents. 

